1. Field of the Invention
This invention relates to a method for adjusting original color signal values for n colors in a color separation system and, more particularly, to a computer implemented method for adjusting the original values based on color interaction.
2. Description of Related Art
In the reproduction of color images, particularly through printing, it has long been known to analyze an original multicolored image, separate the original image colors into a number of primary colors, and to recombine the separated colors in a single colored image to reconstruct the original.
Today, in the most commonly used reproduction processes, the original multicolored image is scanned with a light beam using an image analyze scanner to obtain an electronic signal containing image information in the form of signal intensity level or density variations corresponding to elemental picture areas, known as pixels, of the original picture. The image is scanned through separate sets of colored filters to produce four separate image signals, corresponding to three primary colors and black. The chosen primary colors are typically cyan, magenta and yellow. The output signals are then converted into digital signals and either stored in a memory or used directly to drive an exposing device.
The exposing device may produce black and white film transparencies, corresponding to each of the cyan, magenta, yellow and black color separation signals, known as color separation transparencies or color separations for short. The color separations are used to make printing plates. The printing plates are mounted on a printing press to print, in registration, four overlapping single colored images to reconstruct the multicolored original.
The electronic output signals may also be used to directly drive a color printer to produce a reconstructed multicolored original without ever producing actual color separations or printing plates.
In the process of analyzing and reconstructing the original multicolored image, there are a number of difficulties, some of which have been overcome and some of which remain. One problem is the lack of an exact match between the inks used in printing and the dyes in the original, which is usually a colored photographic transparency or a colored picture. In addition, the filters used to produce the color separations are imperfect. Another problem resides in the printing process itself, whether on a traditional press, an ink jet type device, or a sublimable type printer. This problem is the error of dot gain which is well known in the printing industry. Dot gain refers to the printing of greater or lesser amounts of a given color than what is called for, and results in shifting of the color density and or hues of the colors in the reconstructed image.
It is theoretically possible to correct for the above problems by appropriately modifying the signals used to produce the color separations. However, past efforts to correct such color inaccuracies by modifying the analyze scanner output were only partially successful because they were directed to an analysis of the individual pure color signals, i.e., the magenta, the yellow, the cyan and the black image signals. Each of the signals is corrected for each color individually, using correction coefficients, such as, from a look-up table which are used to modify each signal so that various densities of pure magenta, yellow, cyan and black test patches placed on the original image are reproduced accurately after printing. This approach, however, is somewhat limited since modifying any one of the four color separation signals effects not only the purity of that color, but the hues of each and every color combination in which it is a part thereof. There is, thus, still need for some method to correct the electronic color separation signals prior to printing to accurately reproduce the original hues and color gradations.
It is, therefore, an object of this invention to provide a process for adjusting the output values of color separation data output of a color separation device to more accurately reproduce an original image following printing.